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    Re: Basics of computing sunrise/sunset
    From: Brad Morris
    Date: 2009 Jun 18, 13:49 -0400

    Hi Douglass
    I use a sextant from about 1920.  A wonderful device, identical to the one 
    used by Worsley on his famous boat journey.
    Should I be worried about the filtering qualities of my shade glasses?
    Would there be a way to check to see if my shade glasses are up to the mark?
    Best Regards
    -----Original Message-----
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of douglas.denny{at}btopenworld.com
    Sent: Thursday, June 18, 2009 10:39 AM
    To: NavList@fer3.com
    Subject: [NavList 8696] Re: Basics of computing sunrise/sunset
    NEVER never NEVER look at the sun at any stage of its being above the horizon 
    with any from of magnification:  binoculars, telescopes - anything.
    I have seen 'holes' in the retina with consequent poor central vision due to this in practice.
    Outside of visible spectrum:  especially  infra-red emissions are still there 
    and can damage the fovea - permenantly in seconds if there is any intensity 
    in the sun's light at all.
    Even some so-called "dark glass" or "smoked glass" filters, especially on 
    early sextants are suspect for not filtering out the infra-red sufficiently. 
    They were not aware of the poor quality of filtering with some 'coloured 
    glass filters' outside of visible spectrum in the ninteenth century for 
    example, as the physicists like Tyndal and Brewster were only becoming aware 
    of the physics.
    Make sure you use correct filters whenever using a telescope for viewing the sun.
    Douglas Denny.  Optometrist.
    Chichester. England.
    >> "I recall .... using..... binoculars to spot the moment of sunrise/sunset"
    >>> Readers be warned never to do this if you value your eyesight.
    > As I see it the sun is an orange (elongated horizontally, actually squashed
    > vertically) ball as it is rising or setting, and does not have the magnitude
    > it does higher in the sky thanks to the added atmosphere.  Of course lenses
    > may magnify its "brightness."  At any rate, what we want to look at is when
    > the upper limb is just kissing the horizon (rising or setting) and I do not
    > perceive this as a great danger because of atmosphere, refraction, and the
    > sliver we can see when the body is physically below the horizon. I am open
    > to correction, which is one of the uses of a group like this.
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